The present invention relates to a silicone grease composition, particularly to a silicone grease composition excellent in extreme-pressure (high-load) lubricity.
Silicone oils, particularly dimethyl silicone oils and methyl phenyl silicone oils are so excellent in viscosity-to-temperature relationship, heat and oxidation resistance, shear stability and chemical stability that greases based on these silicone oils also exhibit excellent temperature characteristics, heat resistance, oxidation resistance and chemical stability, thus having been favorably used as lubricants.
However, because the silicone oils are inferior to mineral or synthetic oils in boundary lubricity such as steel-to-steel lubricity, they are unsatisfactory for use under high-speed and high-load conditions and the use thereof is extremely limited. Therefore, there have been made various attempts to improve the boundary lubricity of a silicone oil by adding an oiliness improver or extreme-pressure additive such as a fatty acid or a derivative thereof or a chlorine, fluorine, phosphorus, or amine compound. For example, there have been proposed a process of adding a chlorinated paraffin or a dialkyl chlorendate (see Japanese Patent Publication No. 51-38864) and a process of adding 1,2,3,4,7,8,9,10,13,13,14,14-dodecachloro-1,4,4a,5,6,6a,7,10,10a,11,12,12a -dodecahydro-1,4;7,10-dimethanodibenzo[a,e]cyclooctene (see Japanese Patent Laid-Open No. 62-283196).
However, no satisfactory boundary lubricity has been attained as yet even by these attempts. A grease composition containing as a base oil a silicone oil containing long-chain alkyl groups (see U.S. Pat. Nos. 3,579,467 and 3,673,089) exhibit excellent lubricity under low-load conditions, but it cannot exhibit sufficient lubricity under high-load conditions. Therefore, further improvement in the high-load lubricity has been expected.
Accordingly, it is an object of the present invention to provide a silicone grease composition excellent not only in heat and cold resistance, oxidation resistance, chemical stability and low-load lubricity but also in extreme-pressure (high-load) lubricity.
It has now been found by the inventors of the present invention that the object is accomplished by the addition of an organomolybdenum compound to a silicone grease composition containing as a base oil a silicone oil containing long-chain alkyl groups.